438 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 18 



AGRICULTURAL CHEMISTRY.— T/zr effect of alum on silicate 

 colloids} C. S. ScoFiELD, Bureau of Plant Industry. 



On the irrigated lands of the western United States there are numer- 

 ous instances where the soil contains soluble silicates, chiefly combined 

 with sodium. Where the stronger acids, the sulfates and the chlorides, 

 are removed by the leaching action of irrigation these colloidal silicates 

 cause serious trouble by checking the free movement of water through 

 the soil and by cementing the soil particles into a solid mass when the 

 water is evaporated. 



The complete reclamation of salty irrigated land is often difficult 

 or impossible because of the presence of these colloidal silicates, which 

 cannot readily be leached from the soil. The occurrence of sodium 

 silicate as an important component of the salt complex of arid soils 

 has not been generally recognized, probably because it does not readily 

 leach out of the soil. It is also often confused with sodium carbonate 

 or "black alkali" since both salts give an alkaline reaction with phenol- 

 phthalein. It now seems probable that the well-known puddling 

 effect in soils usually ascribed to sodium carbonate is to be explained 

 as due to sodium silicate. 



In attempting to improve the physical condition of certain irrigated 

 soils that are relatively impermeable to water and that become very 

 hard on drying, it has been found that aluminum sulfate is very effec- 

 tive. This salt is readily soluble and in solution reacts directly with 

 sodium silicate forming an insoluble aluminum silicate and leaving 

 in the soil solution the resulting sodium sulfate which may be removed 

 easily by leaching. 



The results of such laboratory and field experiments as have been 

 made indicate that aluminum sulfate may be used freely on alkaline 

 irrigated land without injury to succeeding crops. In fact an exam- 

 ination of the leachings from soils to which liberal applications of 

 aluminum sulfate have been made shows that practically all the alum- 

 inum is precipitated in the soil and that corresponding quantities of 

 sodium, calcium, and magnesium are released into the soil solution 

 combined with the sulfate radicle. 



In view of the possibility that aluminum sulfate may come to be used 

 extensively in the reclamation of alkali lands under irrigation it is 

 fortunate to find that there are numerous deposits of alum in the 

 western United States. Some of these deposits appear to contain 

 alum in soluble form while in other cases it occurs as alunite which 



1 Received October 10, 1921. 



